1. Field of the Invention
The invention relates to a method of producing a thermoelectric conversion element that contains ceramic that is an insulating material.
2. Description of the Related Art
A thermoelectric conversion material converts thermal energy to electric energy, and converts electric energy to thermal energy. The thermoelectric conversion material is used to form a thermoelectric conversion element that is used as a thermoelectric cooling element or a thermoelectric generation element. The thermoelectric conversion material performs thermoelectric conversion using the Seebeck effect. The thermoelectric conversion performance of the thermoelectric conversion material is represented by the following equation (1) that is referred to as a performance index ZT.ZT=α2σT/κ  (1)
(In the above equation, “α” represents a Seebeck coefficient, “σ” represents an electrical conductivity, “κ” represents a thermal conductivity, and “T” represents a measured temperature).
As evident from the above equation (1), the Seebeck coefficient α and the electrical conductivity σ of the used material need to be increased, and the thermal conductivity κ of the used material needs to be decreased, to increase the thermoelectric conversion performance of the thermoelectric conversion material. Fine particles (inactive fine particles) that do not react with the base material of the thermoelectric conversion material may be added to particles of the starting material of the thermoelectric conversion material, to decrease the thermal conductivity κ of the material. In this case, the inactive fine particles scatter phonons that are mainly responsible for thermal conduction in the thermoelectric conversion material, and thus, the thermal conductivity κ is decreased.
However, in the conventional thermoelectric conversion material, an effect, which is caused by deterioration of property values other than the thermal conductivity κ, such as electrical resistivity, is greater than the effect caused by scattering the phonons using the inactive fine particles. This interferes with improvement of the performance of the thermoelectric conversion material. Therefore, for example, Japanese Patent Application Publication No. 2000-261047 (JP-A-2000-261047) describes a thermoelectric conversion material produced by preparing fine particles of a starting material; evenly dispersing fine particles of ceramic or the like, which do not react with a base material, in the fine particles of the starting material; and performing sintering.
According to the publication No. 2000-261047, because both of the starting material and the inactive fine particles are fine particles, the inactive fine particles are easily dispersed in the entire base material of the thermoelectric conversion material. Therefore, there is a high probability that the inactive fine particle exists between the particles of the starting material. This prevents crystallization of the particles of the base material. Also, the fine particles of the starting material and the inactive fine particles are prepared so that the ratio of the diameter of the fine particle of the starting material to the diameter of the inactive fine particle is approximately 1, that is, the size of the fine particle of the starting material is substantially equal to the size of the inactive fine particle. Therefore, the inactive fine particles are not unevenly dispersed, that is, the inactive fine particles are evenly dispersed in the thermoelectric conversion material. This suppresses the decreases in the other property values, such as the electrical resistivity, due to uneven distribution of the inactive fine particles.
However, because the nano-size particle has a large specific surface area, the nano-size particles are likely to agglutinate due to, for example, the van der Waals' force. Accordingly, when the particles of the thermoelectric conversion material and the inactive fine particles are simply mixed as in the above-described method, the inactive fine particles agglutinate, and accordingly, the inactive fine particles become the micron-size particles. Thus, it is not possible to disperse the nano-size inactive fine particles in the thermoelectric conversion material. As a result, the distance between the inactive fine particles is larger than a phonon mean free path, and thus, the thermal conductivity may not be sufficiently decreased.
In the above-described technology, the other property values, which are not directly related to the above-described equation (1), such as the electrical resistivity, are adjusted by evenly dispersing the inactive fine particles. However, in the equation (1), the electrical conductivity a and the thermal conductivity κ, which are directly related to the performance index ZT, are not examined. Therefore, in the above-described technology, the inactive fine particles are the micron-size particles. Also, the state in which the inactive fine particles are dispersed is not accurately examined.
Carriers (electrons or electron holes) contained in the thermoelectric conversion material transfer both of heat and electricity. Therefore, the electrical conductivity σ is proportional to the thermal conductivity κ. Further, it is known that the electrical conductivity σ is inversely proportional to the Seebeck coefficient α. Accordingly, in general, even if the electrical conductivity σ is increased, the thermal conductivity κ is increased, and the Seebeck coefficient α is decreased. Also, because effective mass is inversely proportional to mobility, if the mobility is increased, the effective mass is decreased.